The narrow-line Seyfert 1 (NLS1) galaxy Mrk 478 displays very broad ultraviolet emission lines with strong blue wings (including O VI, N V, He II, and C IV) in the far-UV spectra obtained with FUSE and HST/FOS, suggesting the presence of a highly ionized outflowing wind. After separating out a “virial” emission component (entire low-ionization lines, the symmetric portion of higher ionization lines centered at zero velocity), we measure line ratios as a function of velocity for the “wind” component. The photoionization code Cloudy is used to model the wind component. For the wind emission between −8000 km s−1 to −2000 km s−1 we find a solution with a very high ionization parameter (log U ~ 0.12−0.53), low gas density (log nH ~ 7.9) and column density (log NH ~ 23). Modelling velocity bins from the wind component indicates that the ionizing photon flux steadily increases with the velocity. This result is not particularly model dependent. Using the photon flux to provide a distance, we determine a kinematic relationship of V ∝ R−0.84±0.20 or V ∝ R−0.61±0.13 for two incident continua, which is consistent with some radiation-driven dynamical models. We conclude that Mrk 478, and by extension other NLS1s with similar emission-line profiles, might provide direct evidence for significant decelerating winds present in at least some active galaxies.